1. Field of the Invention
This invention relates generally to the deposition of silicon-containing films, and more particularly to the use of trisilane, Si3H8, in chemical vapor deposition processes for the deposition of thin silicon-containing films on various substrates.
2. Description of the Related Art
Silane (SiH4) is widely used in the semiconductor manufacturing industry to deposit silicon-containing (“Si-containing”) films by chemical vapor deposition (CVD). However, the deposition of very thin (e.g., about 150 Å or less) silicon-containing films using silane is very challenging, particularly over large area substrates. Very thin Si-containing films deposited using silane are often not continuous, due to island-like film nucleation processes, or have very rough surfaces due to the coalescence of island-like nuclei. In addition, the elemental composition of doped thin films is often not homogeneous in the cross-film and/or through-film directions because of differences in relative incorporation rates of the dopant elements. The resulting films do not exhibit uniform elemental concentrations and, therefore, do not exhibit uniform film physical properties across the surface and/or through the thickness of the film.
Deposition of such thin films with uniform elemental concentrations represents a serious challenge for deposition processes that rely on silane as the silicon source precursor. Typical furnace-based deposition processes that utilize silane are generally unable to deposit continuous, smooth and homogeneous films having a thickness of 100 Å or less. Plasma-enhanced CVD processes typically have serious limitations for the deposition of homogeneous, continuous thin films with thicknesses below about 200 Å. U.S. Pat. No. 5,648,293 states that, for an amorphous silicon layer over a transistor gate insulator, when the film thickness is less than approximately 15 nanometers (150 Å), both decrease of the electron mobility and increase of the transistor threshold voltage appear. Similarly, typical single wafer thermal CVD processes also suffer from an inability to deposit smooth, homogeneous thin film materials with a thickness of 150 Å or less.
Attempts to produce thin Si-containing films and incorporate them into devices have not been entirely satisfactory. For example, U.S. Pat. No. 6,194,237 discloses depositing a conductive layer of Si0.7Ge0.3 on SiO2, depositing another layer of SiO2 over the conductive layer, and then annealing so that the conductive layer forms quantum dots. The conductive layer is stated to have a thickness of 30 Å, but the wide variation in size and distribution for the resulting quantum dots indicates that the conductive layer was not deposited uniformly. Attempts to provide quantum dots of more uniform size and distribution have been disclosed, but typically involve high temperatures and/or more complicated deposition schemes, see e.g. U.S. Pat. No. 6,235,618.
Japanese Patent Application Disclosure No. H3-187215 discloses the use of pure disilane (free of silane and trisilane) in a thermal CVD device to deposit a film having a thickness of 180 Å; see also Japanese Publication No. 03187215 A. U.S. Pat. No. 5,789,030 discloses a low pressure CVD (“LPCVD”) method for depositing an in-situ doped silicon thin film that involves first depositing a very thin layer of silicon before introducing a dopant gas species to form the doped film. While the initial undoped layer is stated to be only several monolayers thick, the overall thickness of the layer is 500 Å to 2,000 Å, including the in situ doped portion.
The use of higher silanes such as disilane and trisilane is sometimes mentioned in the art as an alternative to the use of silane, but in most cases the only data reported concern the use of silane. Disilane (Si2H6) is known to be less stable than silane, and in deposition experiments employing disilane it was reported that disilane gives poor step coverage and that the deposition reaction is too violent to be controlled within the temperature range of 400° to 600° C., see U.S. Pat. No. 5,227,329. Trisilane is even less thermally stable than silane.
The ability to deposit very thin, smooth Si-containing films would satisfy a long-felt need and represent a significant advance in the art of semiconductor manufacturing, particularly for making future generations of microelectronic devices having ever-smaller circuit dimensions.